1. Field of the Invention
This invention relates to a method for authenticating a copied document by comparing a scanned image of the copied document with a scanned image of the original hardcopy document. In particular, it relates to a method for authenticating a copied document that contains both dark and halftone text.
2. Description of Related Art
In situations where a hardcopy document (referred to as the original document in this disclosure) is copied, e.g. by using a photocopier, and the copy is distributed and circulated, there is often a need to determine whether a purported true copy (referred to as the target document in this disclosure) is authentic, i.e., whether the copied document has been altered while it was in circulation. A goal in many document authentication methods is to detect what the alterations (additions, deletions) are. Alternatively, some document authentication methods determine whether or not the document has been altered, without determining what the alterations are.
Various types of document authentication methods are known. One type of document authentication method performs a digital image comparison of a scanned image of the target document with an image of the original document. In such a method, a digital image of the original document is stored in a storage device when the copy is made. Later, the target document is scanned, and the stored image of the original document is retrieved from the storage device to compare with the image of the target document. In addition, certain data representing or relating to the original document, such as a document ID, is also stored in the storage device. The same data is encoded in barcodes which are printed on the copied document when the copy is made, and can be used to assist in document authentication.
With the desire for high quality text and pictures, printed documents are typically scanned at a high resolution, for example, often at 600 dpi (dots per inch) or higher. Thus, document authentication by image comparison can be computationally intensive.
In a printed hardcopy document, gray or light color text or image is often printed using a halftone method. For example, a printed gray area will contain a plurality of black dots of ink or toner, the sizes and/or density of the dots being dependent on the gray-scale value of the gray area. Similarly, light colored areas are printed by printing halftone patterns of color saturated dots. In this disclosure, text printed by a halftone method is referred to as halftone text. Image comparison for documents containing both non-halftone (also referred to as dark or black) text and halftone text may present additional challenge.